Characterization of ozone in the middle troposphere over Japan from 6-year observation at the summit of Mount Fuji (3776m)

Abstract

Tropospheric ozone was continuously measured at the summit of Mt. Fuji (3776 m a.s.l.) for 6 years (1992-1998). Although mountain sites are usually influenced by mountain-valley winds, the small variance of the averaged diurnal ozone variation (0.77 ppbv) indicates that air at the summit of Mt. Fuji was hardly influenced by the boundary layer. Thus, the observations suggest some characteristic features of ozone in the middle troposphere over Japan. Annual variation at the summit was characterized by a bimodal seasonal cycle with maxima in May and October and minima in August and December. The summer minimum, which causes the seasonal cycle to be bimodal, resulted from summertime dominance of ozone-depleted maritime air at the summit. In June, however, air with a low water-vapor mixing ratio, high potential vorticity (PV), and enhanced ozone (> 60 ppbv) was occasionally observed; these characteristics suggest that this air mass originated in the stratosphere or the upper troposphere. The small ozone variation during winter may be due to the suppression of its photochemical destruction and strong zonal winds, which cause the ozone distribution to be zonally homogeneous in the middle troposphere in winter. In addition, the infrequency of ozone intrusions from the stratosphere in winter may also contribute to the small wintertime variation. The annual course of daily mean ozone at the summit was synchronized with that of clear-sky solar radiation from late autumn to early spring, and both minima occurred in late December; this correspondence suggests that solar radiation controls observed ozone levels at the summit during this part of the year. In spring, daily mean ozone increased simultaneously with daily solar radiation, and the ozone concentration was not correlated with PV; this result suggests that the spring ozone maximum results mainly from photochemical ozone production. However, the possibility that indirect stratospheric ozone intrusions or aged stratospheric ozone contribute to the spring ozone maximum cannot be ruled out. The ozone concentration at the summit of Mt. Fuji increased during the 6-year observation period at the rate of 0.49 ppbv year −1 , but the increasing trend was not significant at the 95% confidence level.